1. Field of the Invention
This invention relates to a method of and an apparatus for submerged arc overlay welding of metallic work pieces with metal strips melting in an electric arc. More particularly it relates to an arrangement of electro-magnetic pole pieces which results in improved weld bead surface quality, void free tie-ins between adjacent weld beads, and extremely high coverage rates.
2. Description of the Prior Art
Heretofore, it has been known to plate or clad various members or work pieces by establishing an electric arc between the work piece and a plating material such as a metal strip and by melting the strip in the arc so as to deposit the melted material of the strip on the work piece. It is also known to move the work piece relative to the plating material so as to deposit the melted material in a generally linear plating zone. Such weld cladding is generally performed by the submerged arc process whereby the electric arc and the molten metal are entirely covered by a layer of a suitable weld flux material. A typical application of such a cladding method would be to clad the interior surface of a cylindrical vessel with a corrosion resistant alloy such as stainless steel. Such a cladding operation would be carried out by positioning the welding head adjacent one end of the interior of the vessel to be clad and then simultaneously rotating and linearly translating the vessel so that the resulting weld bead would form a spiral pattern with adjacent weld beads overlapping one another so that they form a substantially uniform thickness covering on the interior surface of the vessel.
Numerous problems have been encountered in developing a technique which permits the deposition of a high quality clad surface using such a strip electrode as the source of cladding material. When the term strip is used herein it is meant to refer to any source of welding material wherein the strip width is substantially greater than the thickness of the strip, typical such strips are in the order of 2 to 6 inches wide and 0.020 to 0.035 of an inch in thickness. Included among the numerous problems encountered are the following: achieving a uniform thickness weld deposit; achieving a substantially void free weld deposit; achieving an acceptable rate of coverage; and, a particularly perplexing problem has been the inability to achieve a void free tie-in between adjacent weld beads having a thickness at least as great as the weld beads themselves. The inability to achieve a satisfactory tie-in between adjacent weld beads often leads to the need to go back and deposit a supplementary quantity of weld material in this region of the clad surface. Such a course is, of course, undesirable because of the additional time and materials needed to carry out such a repair operation.
Many attempts have been made to solve the above recited and other problems which have arisen in the development of submerged arc welding with strip electrodes. However, as the width of the strips used increased, the weld bead surface and the edges of the weld beads deteriorated substantially. As a result, it was impossible to obtain a high quality flawless clad surface since there were noticeable valleys or recesses and slag inclusions occuring at the transition or tie-in point between adjacent weld beads.
The following United States Patents are representative of patents directed towards improving the quality of the resultant cladding in a submerged arc strip cladding process. U.S. Pat. No. 3,584,181 "Method of Arc Welding For Hard Facing" of Nemoto et al; U.S. Pat. No. 3,659,075 "Magnetic Control of Arc in Strip Plating" to Pellkoffer; and U.S. Pat. No. 3,882,298 "Method of and Apparatus for the Submerged Arc Surfacing of Metallic Work Pieces" to Neff et al. The Nemoto patent is directed towards eliminating the malignant effect of the circular magnetic field around the strip electrode caused by the welding current and involves the imposition of a magnetic field through the strip electrode and the work piece in the direction from the electrode to the work. Pellkoffer provides means between the work piece and the electrode strip for creating a magnetic field having a plane of maximum magnetic force passing through the work piece at a point in the plating zone for imposing a lateral force on the electric arc to move the electric arc across the work piece. Neff et al provides for electro-magnetic means producing a stationary magnetic field and having a pair of poles of opposite polarity operatively mounted at opposite sides of the electrode strip. It has been found, however, that even making use of all of the suggested prior art techniques for improving deposition rate and weld bead quality that the weld beads deposited still suffered from non-unformities, inclusions and problems in achieving a sound tie-in of adjacent weld beads. It is towards eliminating these significant problems that the present invention is directed.